Radio receiver



Oct. 13, 1942. JACOB 2,298,297

RADIO RECEIVER Filed June 22, 1940 3 Sheets-Sheet 1 i 20 a? :4 23 i \T E24 T T A JAAAnAnA nnnnn ATTORNEY F. N. JACOB RADIO RECEIVER Filed June22, 1940 Oct. 13, 1942.

3 Sheets-Sheet 2 INVENTORV, *fmderg'muacob-i aBY" ATTORNEY Oct. 13,1942. 2 F. N. JACOB RADIO mcmvm Filed June 22, 1940 3 Sheets-Sheet 3 W 5M Q m H mm 5 1 V m m M R W Q M m j N w Ms Q 8. wag? am L wfl ATTORNEYPatented Oct. 13, 1942 RADIO RECEIVER Frederick N. Jacob, Chicago, 111.,assignor to Johnson Laboratories, Inc., Chicago, 111., a corporation ofIllinois Application June 22, 1940, Serial No. 341,791

22 Claims.

This invention relates to an amplifying system in which a plurality ofvacuum tubes are arranged to selectively amplify a desired highfrequency signal, and to simultaneously improve the performance of theresonant circuits associated with the vacuum tubes. The invention is inthe nature of an improvement on the invention disclosed in applicationSerial No. 339,697, filed on June 10', 1940, in the name of William A.Schaper. In said application a regenerative amplifying system isdisclosed in which the performance of the tunable circuitsof a radioreceiver over the tuning range is substantially improved by providingseparate vacuum tubes for the regenerative and the amplifying functions.

In accordance with the invention one or more resonant circuits areemployed whose performance as to both gain and selectivity issubstantially uniform over the frequency range. Fur thermore, I employcircuits which are, in general, of considerably lower inherent resonantgain and selectivity than the non-uniform circuits heretofore employed,but this deficiency is compensated for by their uniformity and by themeans employed to greatly increase their effective resonant gain andselectivity as hereinafter explained. By using circuits having lowinherent resonant gain, it is possible to employ inexpensive coils inwhich the ratio of L/R. is of a relatively low order, the use of coilswound with special very low resistance windings not being required inorder to secure an adequate amount of gain. The use of such low Q coilsfor the resonant circuits tends to decrease the cost of the receiver.

By employing regeneration, however, the selectivity and gain perresonant circuit are very substantially increased so that it is possibleto secure a degree of selectivity and gain with a single resonantcircuit comparable to that which might be obtained by the use of severalresonant circults of the usual type connected in cascade in a radioreceiver. The number. of resonant circuits required to secure thedesired selectivity and gain is thus reduced, resulting in a furtherdecrease in the cost of construction of the receiver.

By employing regeneration according to my invention, the gain of eachstage may be increased to such an extent that fewer cascaded amplifyingstages are required, thereby resulting in a further decrease in cost ofthe receiver with respect to the number of amplifying tubesrequired.According to my invention, it is possible to use resonant circuits ofinexpensive construction which without regeneration produce a gain perstage of the order of 50 and by a suitable amount of regeneration tobuild up the gem t the order of 500 per stage and, since the total gainis substantially equal to the product of the gains per stage accordingto the number of stages used, it will be seen that by employingregeneration it is possible, by means of my invention, to construct aradio receiver having an adequate amount of gain with only a fewamplifying stages.

My invention comprises the use of a circuit having substantiallyconstant resonant gain and constant selectivity over the frequency rangethrough which it is tunable, such a circuit having a substantiallyconstant ratio of inductance to resistance (L/R) and a substantiallyconstant dynamic resistance (L/RC) This circuit is preferably one havingan inductance coil and 8. capacitor, the inductance being varied bymovement of a compressed comminuted ferromagnetic core relative to theinductance coil to effect its tuning in the manner known as permeabilitytuning. The inductance of such a circuit is inversely proportional tothe square of the frequency and the total resistance must also beinversely proportional .to the square of the frequency in order tosecure a constant dynamic resistance. If a negative resistance isintroduced into such a circuit, as by regeneration, the effectiveresistance of the circuit becomes R,R,' where R is the resistance of thecircuit and R. is the negative resistance, and it is this total oreffective resistance which must now be maintained inversely proportionalto the square of the frequency.

Where the tunable circuit is associated with an amplifying tube, Iintroduce the negative resistance by means of a separate vacuum tube ofthe regenerative type, and which is coupled to the amplifying tube bymeans of a coupling system so arranged as to automatically introduce theproper amount of negative resistance to maintain the dynamic resistanceof the circuit substantially constant. In the above Schaper applicationan arrangement is disclosed for introducing the proper amount ofnegative resistance into the resonant circuit, which is shown connectedto the plate circuit of the amplifying tube. This arrangement comprisestwo capacitive couplings in cascade between the plate and grid circuitsof the vacuum tube which effects the regeneration. Since the mutualreactance due to each of these couplings is inversely proportional tothe first power of the frequency, the total effective mutual reactancebetween the plate and the grid circuits due to these two couplings isinversely proportional to'the square of the frequency.

I have found that when this type of regenerative amplifying arrangementis permeability tuned over an extended range of frequencies, as forexample, over the range from 1600 to 600 kilocycles, the selectivity ofthe resonant circuit tends to decrease by a substantial amount towardthe low frequency end of the tuning range. This results in thetransmission of a substantially wider band of frequencies at this end ofthe range than at the high frequency end with the accompanyingpossibility of receiving the carrier of one or more undesired stationsalong with that of a desired station to which the resonant circuit istuned. It is an object of the present invention to increase theselectivity and gain of the resonant circuit or circuits of anamplifying arrangement of this type and to secure substantially uniformselectivity and gain throughout extended tuning ranges of the circuits.It has been found possible to secure these desirable results byconnecting in the grid-cathode circuit of the regenerative tube aparallel resonant circuit which presents a capacitive reactance at thefrequencies to which the permeability tuned circuit is tunable. Theaddition of such a parallel resonant circuit was found to modify theregenerative effect in such a manner that the width of the band offrequencies transmitted remains substantially constant over the tuningrange and, furthermore, tends to prevent the regenerative amplifyingsystem from going into oscillation.

Where an amplifying stage is used in association .with a regenerativetube arrangement, as just described, in a radio receiver provided withmeans for automaticallycontrolling the gain of the amplifier tubessubstantially inversely as the strength of a received carrier, it wasfound that as the grid bias of the amplifying tubes becomes morenegative, the selectivity tended to increase by a substantial amountand, under certain conditions of initial adjustment, the receiver mightgo into oscillation. It is a further purpose of the present invention toprovide means for compensatingfor this tendency toward increasedselectivity. I have found that by simultaneously in-- creasing the gridbias of the regenerative tube associated with each amplifying tube by anvention, however, reference is made to the accompanylng drawings inwhich like reference characters designate like parts throughout theseveral views, and in which:

Fig. 1 is a schematic circuit diagram of an amplifying stage embodyingthe invention;

Fig. 2 is a schematic circuit diagram of a radio receiver embodying theinvention: and

Figs. 3- and 4 are schematic circuit diagrams of modiiie'dTforms of theregeneration control ar- "rangement of Fig. 2.

Referring to Fig. 1 of the drawings, the basic arrangement, according tomy invention comprises, in the illustrative example shown, a resonantcircuit I, tunable over a range of frequencies, such as the broadcastrange, by a ferromagnetic core 2a movable relatively to the inductivebranch of the circuit, namely, the coil 2. The circuit has two parallelcapacitive branches 3 and 4, 5, one of which comprises the capacitors 4and 5 connected in series. As a means for producing a. substantiallyconstant increase in the dynamic resistance of the circuit I, there isprovided a vacuum tube 6 having its control grid 1 connected through apath of negligible highfrequency impedance through lead 8 andcapaccurrents of the frequencies to be amplified.

Cathode II of vacuum tube 6 is connected to ground through a resistor I6and inductance coil 23 in series, coil 28 being shunted by a capacitor24 to form a parallel resonant circuit having a capacitive reactance tocurrents of the frequencies to which the circuit I is tunable.

The plate circuit of amplifying tube I 6 includes the plate II, variableinductance 2 and voltage source I4, a tap on source I4 supplying asuitable potential to screen grid I8, and a capacitor 20 being connectedto the tap point, to provide a bypass between the screen grid andcathode I9. The input signal is applied to the control grid 2I of vacuumtube I6 and the amplified output is taken ofi through any suitablemeans, herein illustrated as a capacitor 22.

As described in the above mentioned Schaper application, the vacuum tubeI6 functions in the usual manner as a radio frequency amplifier. Itsplate load comprises resonant circuit I, which may be tuned to a desiredfrequency by adjusting the value of inductance 2. The path of highfrequency currents in the grid circuit of vacuum tube 6 may be tracedfrom grid I through lead 8, by-pass capacitor 9, capacitor 5 in parallelwith the series combination of capacitor 4, and inductor 2 and capacitor3 in parallel, and resistor I2 to the cathode II. Thus capacitor 5 iscommon to the grid circuit of tube 6 and to resonant circuit I. Highfrequency currents in the plate circuit of vacuum tube 6 follow a pathwhich comprises capacitor 4 in parallel with the series combination ofcapacitor 5, and inductor 2 and capacitor 3 in parallel, and resistorI2. Thus capacitor 4 is common to the plate circuit of tube 6 and toresonant circuit I. The grid and plate circuits are, therefore, eachcapacitively coupled to resonant circuit I, and it. is these twocascaded couplings which cause regeneration by the tube 8 and cause theamount of regeneration to vary automatically with the frequency.Resistor I2 functions to improve the stability of vacuum tube 6.Resistor I5 and coil 23 provide a return path for the direct currentcomponent of the cathode current of tube 6 to follow. Due to the effectof regeneration in tube 6, the efiective resistance of circuit I isgreatly reduced, with the result that its resonant gain is considerablyincreased and it provides a much higher degree of selectivity than wouldotherwise be secured from it.

7 The degree of regeneration at any given frequency depends upon theamount of coupling between the plate and grid circuits of the tube 6 andmay be varied by adjusting the value of resistor I2, the regenerationincreasing as the value of this resistor is decreased. The amount ofcoupling depends on the relative values of capacitors 3, 4 and 5, thecoupling increasing as capacitors 4 and 5 in series are made larger incomparison with capacitor 3.

The variation of the gain and selectivity of the amplifying stage as thecircuit I is tuned over a range of frequencies depends partly on thecharacteristics of variable inductance 2, that is, the relation of itsinductance to its radio frequency resistance over the tuning range. Theinductance device is preferably such that the ratio of the inductance tothe high frequency re sistance of the resonant circuit remainssubstantially constant over the tuning range. The defrequencies when theresonant circuit is tuned over an extended frequency range, for example,the broadcast range, and more particularly when the capacitors 4 and 5are small with reference to capacitor 3, due to the shunting eifect ofreslstors l2 and on the capacitor 5. This tendency is overcome, inaccordancewith my invention, by the action of the parallel resonantcircuit comprising inductance coil 23 and capacitor 24. This circuit isdesigned'to present a capacitive reactance eflectively in series betweencathode Ii and the low potential terminal of circuit I, at thefrequencies to which the circuit l.is

' tunable. In an embodiment of the invention in which the resonantcircuit i was tunable over the frequency range of from 600'to 1600kilocycles, the constants of the circuit elements 23, 24 were sodesigned as to give this circuit a resonant frequency of a proximately330 kilocycles. By properly proportioning the constants of circuit23-24, furthermore, it is possible also to compensate for a variableinductance 2 which does not in itself provide constant circuitselectivity throughout the tuning range, in such a way thatsubstantially constant selectivity of the system is obtained.- Thisadditional refinement of performance is one of the features of myinvention.

Although a tetrode and a triode are shown in Fig. 1, it will beunderstood that they were chosen for illustrative example only, and thatother types of ,vacuum tubes may equally well be employed. For example,the amplifying tube may be a pentode having a suppressor grid in addi--tion to the other grids shown. Furthermore, the elements of the tubes 6and I6 maybe mounted within a single envelope and mounted on a singleSuch a combination tube may employ a base. single heater to heat boththe cathodes II and it, or separate heaters connected in series or inparallel may be employed. a

When in the claims, therefore, I describe my novel structure asincluding vacuum tubes, it will be understood that the essentialelements of these tubes may be incorporated in a single evacuatedenvelope with the possible elimination of some of the elements whichwould be required in the case of a plurality of separate envelopes. For

example, it-the several elements were arranged in a single evacuatedenvelope, only a single heater for the several cathodes might beprovided. An embodiment of the invention, in a radio receiver tunableover the broadcast band, is shown in Fig. 2. the receiver comprisingseven vacuum tubes arranged as follows: two radiofrequency amplifyingtubes 25 and 26 arranged in cascade, two regenerative vacuum tubes 28and 20, cooperating respectivelywith the reso nant; circuits 30, acombined detector, first audio frequency amplifier and automatic volumecontrol tube iii, a power output vacuum tube 32, and a rectifying vacuumtube 33. with the exception of the novel arrangementsnow to be described, the receiver is of conventional design and operates in theusual manner.

The signal energy is supplied to the control grid 34 of tube 25 from thesecondary winding 35' of a transformer 36 whose primary winding isconnected to the antenna A. The secondary winding 35 is shunted by anadjustable capacitor 31 to form a circuit 38 tunable to the desiredcarrier frequency, the inductance of secondary winding 35 being variedby means of'a movable ferromagnetic core 39. The voltage developedacross resonant circuit 38 is applied to control grid 34 of tube 25 bymeans of a capacitor 40, a

suitable bias voltage being supplied to grid 34 through a resistor 4|,this bias voltage being filtered by means of the resistor 42 andcapacitor 43. A capacitor 44 is shunted between control grid 34 andground, and the resistor 45 connected between the tube cathode 46 andground provides the normal grid bias voltage, this resistor beingshunted by a capacitor 41. Screen grid 49 is supplied with a suitablepositive potential through the resistor 50 and is by-passed to groundby-the capacitor ii.

The plate circuit of amplifying tube 25 includes the resonant circuit30, which corresponds to the circuit I of Fig. 1 and comprises theadjustable capacitor 52 which is shunted across the inductance coil 53and capacitors 54, 55, the inductance of the circuit being controlled bythe movable ferromagnetic core 29. Plate voltage for tubes 25 and 28 issupplied through the resistor 56, which is by-passed toground throughthe capacitor 61. Grid 48 of tube 28 is supplied with direct currentbias voltage through a resistor 58 and is maintained at ground potentialwith reference to radio frequency by the capacitor 58. Cathode 60 isgrounded through resistor 6i and inductance coil 62 in series, the coil62 being shunted by a capacitor 63 to form a parallel resonant circuit.

connections between the tubes 28 and 29 aresimilar to those between thetubes 25 and 28, like circuit components being designated by the samereference characters. The ferro-magnetic cores 39 are preferablyconnected together for simultaneous operation by means of a singlemanual control, as indicated by the dash lines.

The amplified signal-voltage which is developed across the secondresonant circuit 20 is applied to the diode anodes 86 of vacuum tube 3|by means of the coupling capacitor 65, the rectified signal voltageappearing across the resistors 61, i8 and 69 which are connected inseries between the anodes 66. and the cathode 10 of tube 3|. A capacitorH is shunted across the series connection of resistors 68 and 69. Thedirect current voltage developed across resistors 68 and 68 in series isapplied to the control grids 34 of tubes 25 and 26 through the timedelay network comprislng resistor 12 and capacitor 13 and the filterscomprising resistors 42 and capacitors 43. The direct currentbias-voltage developed across resistor '68 is applied to the grids 48 ofthere- ,generative tubes 28, 29 through a time delay network comprisinga resistor 14 and capacitor 15 and the filters comprising the resistors58 and capacitors 59. The modulation voltage developed across resistors88 and 69 in series is applied to grid 18 of tube 3| by means of acircuit comprising a capacitor 11 and potentiometer '18. A suitablenegative bias voltage is applied to grid 18 through a. resistor 19, thisbias voltage being filtered by the capacitor 88. Also, as shown, thecathode III of tube 3| is grounded.

The audio frequency voltage developed across load resistor 8| in theplate circuit of .tube 3| is applied to the control grid 82 of powertube 32 by means of coupling capacitor 83 and resistor 84. Thepotentiometer 85 serves as a grid leak for grid 82 and also, inconjunction with a capacitor 86, as a tone control device for adjustablyattenuating the higher audio frequencies. A self biasing resistor 81serves to connect the cathode of tube 32 to ground, this resistor beingshunted by the usual capacitor 88. Theplate circuit of power tube 32 iscoupled to any suitable translating device as, for example, a loudspeaker 89 by means of a transformer 80.

Suitable operating potentials are supplied to the several tubes of thereceiver from a source which comprises the rectifier tube 33, multiplewinding transformer N, a filter circuit of the usual type, as shown, anda voltage divider comprising the resistors 82, 93, 94 and 85, which areconnected in series. It will be understood that the heaters of tubes-29, 3| and 32 are energized by winding 98 of transformer 8 I, theheaters and their connecting circuits being omitted from the drawing forthe sake of clarity. One side of the heater circuit is preferablygrounded, as shown.

In the operation of the receiver, a modulated radio frequency signalvoltage impressed on antenna A is amplified successively by the radiofrequency amplifying tubes 25 and 26 operating in conjunction with theresonant circuits 38, 38 and 38, each of which is tuned to resonancewith the incoming signal frequency. and regenerative tubes 28 and 28.This tuning is preferably accomplished by moving the cores 39simultaneously with reference to the inductance coils and 53, but it iswithin the scope of my invention to provide separate actuating means forone or more of the cores 88. The amplified signal current isdemodulated-by the diode portion of tube 3| and the resulting audiofrequency signal voltage is amplifled by the triode' portion of tube 3|and tube 32 from which it is supplied to the loud speaker 89 by thetransformer 88. That portion of the rectified carrier voltage whichappears across diode load resistors 88 and 69 in series is utilized toconquency depends upon the transconductance of tube 28 and the amount ofcoupling between its plate and grid circuits. The amplification of tube28 may be varied by changing its grid-cathode potential, theamplification and hence the degree of regeneration increasing as thispotential is decreased, that is, as the grid is made less negative withreference to the cathode. The amount of coupling between its grid andplate circuits detro] the amplification of amplifying tubes 2.5 and:

pends on the relative values of capacitors 54 and 55 where all otherconstants of the tuned circuit remain fixed, the coupling decreasing ascapacitor 54 is made larger in proportion to capacitor 55.

The amplification of tube 25 is controlled by varying its control-gridbias voltage. Increasing the negative bias voltage causes the plateresistance of the tube to increase by a substantial amount. Since its,plate resistance is effectively in shunt with resonant circuit 30 which,due to the regeneration produced by tube 28, has a relatively highresonant impedance, this variation in plate resistance of tube 25 withvariation of its control grid bias voltage has an appreciable infiuenceon the selectivity and resonant gain of circuit 30. Upon increasing thegrid bias voltage, the transconductance decreases but the plateresistance of tube 25 increases, causing an increase in the selectivityof circuit 30 possibly sufficient to produce sustained oscillationsthrough the action of tube 28. This highly undesirable effect isavoided, in this embodiment of the invention, by applying to grid 48 oftube 28 only a portion of the bias voltage which is applied to thecontrol grid 34 of tube 25, namely, only that portion of the biasvoltage which is developed across resistor 69. In this manner, theregenerative effect of tube 28 upon resonant circuit 30 is decreasedsumciently to compensate for the increase in selectivity due to theincrease in the plate resistance of tube 25. In case of exactcompensation, the selectivity of the system remains substantiallyunchanged as the bias voltage, which is applied to grid 34 of tube 25 bythe automatic volume control arrangement described, is varied over thefull range.

The compensation may be carried further, however, if desired, to providefor a decrease of selectivity under conditions of a large bias voltageas,for example, during the reception of extremely strong signals merelyby applying to grid 48 a larger portion or even all of the biasingpotential applied to the grid 34 of pentode tube 25.

Because the reactance of capacitor 55 increases as the frequencydecreases, the gain and selectivity tend to become appreciably worse atthe lower frequency end of an extensive tuning range, as, for example,the broadcast range due to the shunting eifect of resistors 6| and 66.This tendency is overcome by the action of the parallel resonant circuitcomprising coil 52 and capacitor 83 which is designed to present acapacitive reactance effectively in series between cathode 60 and groundat the frequencies to which circuit 88 is tunable. By properlyproportioning the constants of the circuit 62-63, it is also possible tocompensate for an inductance device 3538 which does not in itselfprovide constant circuit selectivity throughout the tuning range, insuch a way that substantially constant selectivity of the system isobtained.

Measurements were made of the performance of a stageof the receivershown in Fig. 2 comprising the tubes 25 and 28 and their associatedcircuits to determine the effect of the addition of the resonant circuit62-68 on the selectivity of the tunable circuit 30 as it was tuned overa frequency range from 1600 kc. to 600 kc. The results of these testsshow clearly that a Considerable improvement in the selectivity and gaintoward the low frequency end of the range is secured by the addition ofthe circuit 62-63 in the cathode return. By suitable choice of theconstants of this circuit, it was found possible to secure asubstantially constant and high order of selectivity of th tunablecircuit throughout its tuning range without causing the system to gointo oscillation.

Measurements were also made to determine the effect of the change ingrid bias voltage of the triode grid 48 other than the self biasdeveloped across resistor 6|, on the selectivity of the tuned circuitfor different bias voltage of the pentode grid 34. Denoting the biasvoltage of grid 34 other than that present across resistor 45 by VI andthat of grid 48 other than that present across resistor Si by V2, thefollowing results were secured at an applied signal frequency of 1600 Inthe tables the band width of the response curve was measured at a heightequal to one tenth of the peak output voltage. The results indicateclearly that by the application of a grid bias voltage to theregenerative tube grid which is directly proportional to that applied tothe .amplifying tube grid, it is possible to secure a substantiallyconstant degree of selectivity for different amplifying values of thepentode. It is thus possible by tapping oil on the automatic volumecontrol resistor (68-69, Fig. '2) at the proper point to secure aconstant band width or an increasing or decreasing band width depend-;ng upon the amount of automatic volume control voltage utilized.

The circuit arrangement of Fig. 3 is similar to that of Fig. 2 exceptthat the regenerative tube 91 includes a separate diode portioncomprising a pair of auxiliary anodes 98 and a cathode 98a which isseparate from the cathode 91a of the triode portion of the tube. Thesignal voltage is applied to the anodes 93 by means of a capacitor 99and the anodes 98 are connected to the diode cathode 98a by means of theresistor I00. It followsthat the direct current voltage which isdeveloped across the load resistor I is proportional to the strength ofthe signal voltage and this voltage is applied to the grid ml of thetriode portion through the resistor I02, the capacitor I03 serving tofilter out the alternating-component of the bias control voltage. Bymeans of this arrangement and that disclosed in Fig. 4, each stage ofthe receiver supplies its own source of bias potential for the grid ofthe regenerative tube and is thus made independent of external sistorGI, a direct current bias voltage is applied to the auxiliary anodes 98through the resistor I00 in addition to the radio frequency voltageapplied through the capacitor- 99. This results in a delay in theapplication of the automatic control voltage applied to the grid IOI ofthe triode portion of the tube upon a change in the strength-of areceived carrier.

While I have shown my invention in the particular embodiments abovedescribed,.-I ,do not limit myself thereto as I may employ equivalentsthereof without departing from the scope of the appended claims.

Having thus described my invention, what I claim is:

1. A selective high frequency resonant system comprising, incombination, a resonant circuit containing inductance and capacity andhaving an effective resistance, said circuit being provided with meanswhereby the inductance and eflective resistance may be variedsubstantially in proportion to each other, and means for increasing theselectivity of said circuit comprising a regenerative tube having itsanode coupled to said circuit and a circuit connecting the grid andcathode of said tube and comprising a parallel resonant circuit.

2. A selective high frequency resonant system comprising, incombination, a resonant circuit containing inductance and capacity andhaving an effective resistance, said circuit being provided I with meanswhereby the inductance and effective resistance may be variedsubstantially in p'ropor' tion to each other, and means for increasingthe selectivity of said circuit comprising a regenerative tube havingits anode coupled to said circuit and a circuit connecting the grid andcathode of said tube and comprising a parallel resonant circuit disposedin the plate circuit of, the regenerative tube.

3. A selective high frequency resonant system comprising, incombination, a resonant circuit containing inductance and capacity andhaving an effective resistance, said circuit being provided with meanswhereby the inductance and efiective resistance may be variedsubstantially in proportion onto each other, and means for increasingthe selectivity of said circuit comprising a regenerative tube havingits anode 'coupled to said circuit and a circuit connecting the grid andcathode of said tube and comprising a parallel resonant circuit having acapacitative reactance at the frequency to which the circuit containinginductance and capacity is reson'ant.

4. A selective high frequency resonant system comprising, incombination, a resonant circuit containing inductance and capacity,means for tuning said circuit over a range of frequencies, aregenerative vacuum tube device, means for sup plying energy from saidregenerative tube to said resonant circuit, a circuit connected to thecathode of theregenerative tube and in shunt across at least a portionof the capacity of the resonant circuit whereby the selectivity of theresonant circuit tends to decrease as it is tuned toward the lower endof its frequency range and means for compensating for such decrease inselectivity comprising a parallel resonant circuit connected in serieswith said shunt circuit.

5. A selective high frequency resonant system comprising, incombination, a resonant circuit containing inductance and capacity,control means for adjusting the inductance of said circuit to tune itover a range offrequencies, a regenerative vacuum tube device, means forsupplying energy from said regenerative tube to said resonant circuit,said means being arranged to automatically decrease the energy suppliedas said control means is adjusted to tune the resonant circuit towardthe low frequency end of the tuning range, and means for automaticallyincreasing the energy supplied by the regenerative tube to the resonantcircuit in response to the tuning,of the resonant circuit toward the low;-;;;frequency end of its tuning range by the ad- Justment of saidcontrol means to thereby com- {f pensate for the decrease-in energysupplied to the resonant circuit by the regenerative tube. ,,-6,Aselective high frequency resonant system said control means is adjustedto tune the resonant circuittoward the low frequency end of the tuningrange, and means for automatically increasing the energy supplied by theregenerative tube to the resonant circuit in response to the tuning ofthe resonant circuit toward the low frequency end of its tuning range bythe adjustment of said control means to thereby compensate for thedecrease in energy supplied to the resonant circuit by the regenerativetube, said energy increasing means including a. parallel resonantcircuit connected to one of the elsetrodes. of the regenerative tube,the reactance of said parallel resonant circuit being capacitative atthe frequencies within the tuning range of the .resonant circuit.

'7. In combination, a resonant circuit having resistance and comprisinga coil and a capacitor, means for varying the inductance of said coil totune'said circuit over a range of frequencies, an electron dischargetube having coupled grid and .plate circuits and a parallel resonantcircuit which is common to the grid and plate circuits. and means forcapacitively coupling one of said coupled circuits to said resonantcircuit in such a manner that the degree of coupling therebetween isautomatically varied to maintain the ratio between the inductance andresistance of said resonant circuit substantially constant as dynamicresistance being substantially constant throughout the range of saidtunable resonant circuit.

9. A selective high frequency resonant system including a tunableresonant circuit having a relatively low but substantially constantdynamic resistance and having a portion of its capacitance arranged toprovide a midtap, and means for producing a substantial increase in saiddynamic resistance including a vacuum tube having a cathode, a parallelresonant circuit connected between said cathode and the low potentialterminal of said tunable circuit, a direct connection between the anodeof said tube and the high potential terminal of the tunable circuit, anda resistor connecting the tube cathode to said'midtap, said increase indynamic resistance being substantially constant throughout the range ofsaid tunable resonant circuit. a

10. A selective high frequency amplifying system comprising, incombination, an amplifying vacuum tube and a regenerative vacuum tube,each of said tubes having an anode, a cathode and a grid, a resonantcircuit tunable over a range of frequencies and having its highpotential terminal directly connected to the anode of said tubes, alowimpedance connection between the lowpotential terminal of saidresonant circuit and the cathode of said amplifying tube, meansconnected between the cathode of the regenerative tube and the lowpotential terminal of the resonant circuit including a parallel resonantcircuit having a. capacitive reactance at the frequencies to which theresonant clrcuit'is tunable, a direct current connection from anintermediate point of the tunable circuit to the cathode of theregenerative tube, and means for applying negaaive bias voltage to thegrids of said tubes which is proportional to the strength of thereceived signal current.

11. A selective high frequency system comprising, in combination, aresonant circuit comprising an inductive branch, a ferromagnetic coreadjustable with reference to said inductive branch to tune said circuitover a range'of frequencies. means shunted across a portion of saidcircuit acting to decrease the selectivity thereof toward one end of thefrequency range, a regenerative tube having its plate connected to thehigh potential terminal of said resonant circuit, and a circuitconnected between the cathode of the reenerative tube and the lowpotential terminal l of the tunable circuit, the impedancecharacteristic of said circuit being such as to increase the selectivityof said resonant circuit as it is tuned toward said one end of thefrequency range.

12. A selective high frequency amplifying system comprising, incombination, an amplifying vacuum tube and a regenerative vacuum tube.each of said tubes having an anode, a cathode and a grid, resonantcircuit tunable over a range of frequencies and having its highpotential terminal connected to the anodes of said tubes, meansconnected between the cathode of the regenerative tube and the lowpotential terminal of the resonant circuit tending to cause an increasein the selectivity of the resonant circuit as it is tuned toward one endof the frequency range, a connection between an intermediate point ofthe tunable circuit and the cathode of the regenerative tube, and meansfor simultaneously and proportionally reducing the gain of theamplifying tube and the amount of regeneration in the regenerative tube.

13. A selective high frequency amplifying system comprising, incombination, an amplifying vacuum tube and a regenerative vacuum tube,each of said tubes having an anode, a. cathode and a grid, a resonantcircuit tunable over a range of signal frequencies and having its highpotential terminal connected to the anodes of said tubes, meansconnected between the cathode of the regenerative tube and the lowpotential terminal of the resonant circuit tending to cause an increasein the selectivity of the resonant circuits as it is tuned toward thelow frequency end of the tuning range, and means for applying negativebias voltstant throughout the range of saidtunable resage to the gridsof said tubes which is proportional to the strength of the receivedsignal-current.

14. A selective high frequency amplifying system comprising, incombination, an amplifying vacuum tube and a regenerative vacuum tube,each of said tubes having an anode, a cathode and a grid, a resonantcircuit tunable over a range of signal frequencies and havingits highpotential terminal connected to the anodes of said tubes, meansconnected between the cathode of the regenerative-tube and a lowpotential terminal of the resonant circuit tending to cause an increasein the selectivity of the resonant. circuits as it is tuned toward thelow frequency end '01 the tuning range, and means for applying negativebias voltage to the grids of said tubes which is proportional to thestrength the received signal current, said last named means beingarranged to apply a larger negative bias voltage to the grid of theamplifying tube than to the grid of the re generative tube.

15. Means for amplifying received signal currents comprising, incombination, a resonant circuit tunable over a range of frequencies, aregenerative vacuum tube having a cathode, a grid, an anode and a diodeplate, a connection between said anode and the high potential terminalor the resonant circuit, connections respectively between said grid andsaid cathode and different and lower potential points of said resonantcircuit, means coupling the resonant circuit to the diode plate, acircuit conductive to direct current connected between said cathode andthe diode plate, and means for controlling the voltage of said grid inaccordance with the strength of the direct current passing through saidlast named circuit.

16. A selective high frequency resonant system including a resonantcircuit tunable over a range of frequencies and having a substantiallyconstant dynamic resistance, means for producing a substantial increasein said dynamic resistance including a vacuum tube having an anodeconnected to the high potential terminal of said circuit and a parallelresonant circuit connected between the cathode of said tube and the lowpotential terminal of said resonant circuit, said parallel resonantcircuit having a natural resonant frequency outside of the range offrequen cies over which said first named resonant circuit is tunable,said increase in dynamic resistance being substantially constantthroughout the range of said tunable resonant circuit.

17. A selective high frequency resonant system including a resonantcircuit tunable over a range of frequencies and having a substantiallyconstant dynamic resistance, means for producing a substantial increasein said dynamic resistance including a vacuum tube having an anodeconnected to the high potential terminal of said circuit and a parallelresonant circuit connected between the cathode of said tube and the lowpotential terminal of said resonant circuit, said parallel resonantcircuit having a natural resonant frequency outside of the range offrequencies over which said first named resonant circuit is tunable, thenatural resonant frequency of said parallel resonant circuit being lowerthan the lowest frequency of the range over which the first namedresonant circuit is tunable, said increase in dynamic resistance beingsubstantially cononant circuit.

18. The method of increasing theselectivity of a resonant circuittunable overarange of frequencies which comprises the steps of feedingenergy. from a. vacuum tube into said circuit in phase with the currenttherein in such a manner that the gain in selectivity of the circuit isappreciably greater at one end of its tuning range than at the other endthereof and introducing a capacitive reactance betweenanelectrode of thetube and a terminal of the tunable circuit, the magnitude of saidreactance being such as to cause a substantial increase in theselectivity of the circuit at said other end of its tuning range.

19. A radio frequency amplifier system, including an amplifier tube, aresonant circuit con,- nected with said tube, a second tuberegeneratively associated with said resonant circuit and having acontrol electrode, a variable bias system connected with said amplifiertube and developing voltage to control said amplifier tube, and aconnection between said bias system and the control grid of said secondtube substantially maintaining a desired selectivity of said resonantcir cuit for variation in the voltage developed in said bias system.

20. Ar'adio frequency amplifier system, including an amplifier tube, aresonant circuit connected with said tube, a second tube regenerativelyassociated with said resonant circuit and having a control electrode, avariable bias system connected with said amplifier tube-and developingvoltage to control said amplifier tube, and a connection between saidbias system and the control grid of said second tube substantiallymaintaining a desired selectivity of said resonant circuit for variationin the voltage developed in said bias system, said connections with saidbias system being such as to impress on the control electrode of saidsecond tube a voltage smaller than the voltage impressed by said biassystem on said amplifier tube.

21. A radio frequency amplifier system, includ'-' ing an amplifier tube,a resonant circuit con-- nected with said tube, a second tuberegeneratively associated with said resonant circuit and having acontrol electrode, an automatic volume control system connected with andcontrolling said amplifier tube by voltage variation, and a connectionbetween said automatic volume control system and the control grid ofsaid second tube for applying to the control grid of said second tube avoltage less than and proportional to the voltage applied by saidcontrol system to said amplifier tube.

22. A radio frequency amplifier system, including an amplifier tube, aresonant circuit connected with said tube, a second tube regenerativelyassociated with said resonant circuit and having a control electrode, anautomatic volume control system connected with and controlling saidamplifier tube by voltage variation, and a connection between saidautomatic volume control system and the control grid of said second tubesuch as to apply to the control grid of said second tube a controlvoltage of an amount and kind substantially compensating forregenerative change otherwise incident to applying automatic volumecontrol voltage to said amplifier tube.

FREDERICK N. JACOB.

y CERTIFIECA'TE OF CORRECTION. Patent No. '2,298,297.- October. 15, 19m.

FREDERICK N. JACOB.

It is hereby certified that error appears in the printed specificationofthe abo-ve numbered patent requiring correction as follows: Page 2,second column, line 52, for pathbf read -path for-; page 5, secondcolumn,

line lfl, claim 5, for "onto" read --to--; page 6, first column, line16, claim 6, after "capacity" strike out the period and insertinsteadacomm'a;

and second column, linel+9,'cle1m l2,'before "resonant" insert --e.--;and that the said Letters Patent should be read with this correctiontherein that the sange may conform to the record of the case in thePatent Office.

Signed and seal'ed this 1st d y of December, A. D. 19u2.

' Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

